1. Field of the Invention
The present invention relates to a power system compensator apparatus, and power converter apparatus and, more particularly, to a power system compensator apparatus and power converter apparatus employing a static power converter.
2. Description of the Related Art
FIG. 3 is an example of the construction of the known static power system compensator apparatus. As illustrated in FIG. 3, the apparatus comprises a transmission system 1, a receiving system 2, buses or bus-lines 3a and 3b, a transmission line TL 30, a main transformer 4, a transformer for use in a power converter (multi-transformer) 5, primary and secondary windings 5a and 5b of the transformer 5, and a converter 6 such as an inverter. The converter 6 is composed of per phase converters 6a, 6b and 6c and a link capacitor 7. Reference number 8 designates pulse-width modulation (PWM) control means for controlling the converter 6. The PWM control means 8 is constructed of a PWM control unit 8a, a current control unit 8b for controlling active current Ip and reactive current Iq, and an operational unit 8c for computing active current Ip and reactive current Iq. The apparatus further comprises link voltage pickup means 9 for picking up and detecting a link voltage V.sub.LINK, a link voltage control unit 10 for controlling the link voltage V.sub.LINK, current pickup means 11 for picking up and detecting a line current I.sub.L and for outputting it to the operational unit 8c, AC voltage pickup means 22 for picking up and detecting an alternating current voltage V.sub.TL, absolute value pickup means 23 including of a rectifier and other components for picking up and then outputting the amplitude or absolute value of the AC voltage V.sub.TL, and a control unit 28 for controlling the amplitude of the line voltage Vac or reactive power Q output by the absolute value pickup means 23.
In FIG. 3, the converter 6 is a PWM inverter, which is controlled by a PWM control signal provided by the PWM control unit 8a. The PWM control unit 8a comprises a PWM modulator that generates the PWM control signal based on three phase instantaneous voltage command waveforms Vu,v,w (modulating waves). The instantaneous voltage command waveforms Vu, v, w are given as the output of the current control unit 8b that controls the active current Ip and reactive current Iq. The current control unit 8b receives a reactive current command I*q from the control unit 28 that controls the line voltage Vac or the reactive power Q. The current control unit 8b also receives an active current command I*p from the link voltage control unit 10 that controls the link voltage V.sub.LINK. In this known static power system compensator apparatus, the transformer 5 for use in a power converter is not provided with phase-shifting transforming function, thus, the transformer 5 gives PWM voltages in the secondary winding 5b independently and at different times.
In the known compensator apparatus, the input and output of the active current Ip, therefore, the active power P is controlled so that the DC link voltage V.sub.LINK is constant. Under the condition of the constant V.sub.LINK, the AC output voltage, reactive current Iq and reactive power Q of the converter 6 are controlled through a PWM technique (pulse-width modulation technique). Namely, control is performed by changing the amplitude (or the magnitude of the instantaneous waveform) of the modulating wave in PWM. This increases the switching frequency of the converter 6, causing both switching loss and snubber loss to increase, and consequently lowering the efficiency of the apparatus. An increase in the switching frequency requires the recovery of snubber energy if a heavy-duty GTO converter is used as the converter 6. This complicates the design of the apparatus and thereby lowers the reliability of the apparatus. When a pulse-width modulated voltage is applied to the transformer 5, an increased transformer loss results. In addition to the switching loss and snubber loss, the transformer loss lowers the efficiency even further.
In the known compensator apparatus, the transformer 5 is not of the phase-shifting transformer type. Thus, if PWM phase-shifting is attempted in the PWM inverter, which constitutes the converter 6, to lower the level of harmonics, the phase difference appears in the fundamental wave voltage. Since the same phase current (in-phase current) flows through the primary windings 5a which are connected in series, the phase difference in the fundamental wave causes an unbalanced power condition. This causes an unbalanced current in the DC link side of the converter 6. As a result, a series connection cannot be made.
The primary windings 5a may be set up as a multi-transformer type, phase-shifting transformer. In this case, however, if phase shifting is performed on the primary side that handles a high voltage, the connection of the primary windings 5a which have insulation difficulty is complicated. Namely, the primary windings 5a have the complicated connection and insulation difficulty at the same time. Thus, the multi-transformer type, phase-shifting transformer cannot be adopted. The above-described problem remains to be solved. Therefore the static power system compensator apparatus suffers in economic and reliability point of view.